As is generally known, magnetic resonance (MR) imaging has several unique attributes that can be exploited for guiding minimally invasive interventions. Among these include (a) an inherent high tissue contrast with the ability to manipulate contrast differences using various pulse sequences (or contrast agents), (b) a multi-planar imaging capability, and (c) the avoidance of ionizing radiation. In addition to detailed anatomic information, MRI technologies also provide useful physiological information on blood flow, tissue perfusion and diffusion (also referred to as “functional imaging”), and temperature changes in the imaged subject that can be used for monitoring responses to therapy and interventional techniques.
The poor visibility and tractability of interventional devices, such as puncture needles, guidewires, catheters, and the like, is a major problem encountered during MRI guided interventional procedures. While these interventional devices produce an excellent contrast in x-ray images, this is generally not the case during MR imaging. The high MR signal of surrounding tissue masks small interventional devices, making them nearly invisible in MR images.
Several methods have been proposed for tracking and visualizing interventional devices within the MRI environment. These methods for tracking and visualising interventional devices in the MRI environment can be classified as either passive or active methods. In general, MR image contrast generated with passive visualisation methods has proven insufficient to determine the exact spatial position of an interventional device within three-dimensional space, especially when the interventional device is relatively small such as when the device is a needle or a catheter. Consequently, prior art passive methods are not acceptable for guiding the imaging plane with respect to an instrument's location, such as required for interventional device tracking and visualization during a medical or surgical interventional procedure (See for example, Bernstein et al., Magn. Reson. Med., vol. 39:pp. 300-308, 1998; Bakker et al., Magn. Reson. Med., vol. 36:pp. 816-820, 1996).
On the other hand, active determination of the spatial position of an interventional device has proven more promising, and active tracking and visualization techniques have already been applied in several interventional procedures for the purposes of tracking and navigating the interventional device. These prior art active tracking and visualizing methods use MR micro coils or optical navigation systems to provide the spatial coordinates of the interventional device in the MR environment; therefore, these active methods do not rely on vague susceptibility artifacts created by the device itself within the MR image, such as occurs during passive tracking and visualization procedures.
The principle of the prior art active methods, which use one or more MR micro coils, is based on the spatially dependent frequency encoding of an MR signal induced within the micro coil by means of linear magnetic field gradients (See for example, Dumoulin et al., Magn. Reson. Med., vol. 29:pp. 411-415, 1993; Wildermuth et al., CVIR, vol. 21:pp. 404-410, 1998; Kee et. al., JVIR, vol. 10:pp. 529-535, 1999). The MR micro coil detects the local resonant frequency of spins exposed to gradient fields, which is subsequently decoded to determine the position of the MR coil. The orientation of the interventional device in three-dimensional space is determined from the judicious placement of two or three independent MR coils attached to the interventional device.
However, active navigation based on MR micro coils has several drawbacks when used within the MR environment. First, these active techniques require several additional receiver amplifiers and channels for each MR micro coil. Furthermore, the MR imaging sequence used to visualize the surrounding tissue has to be interleaved with a dedicated projection sequence required to assess the position of the MR micro coils as described in U.S. Pat. No. 5,307,808 to Dumoulin et al., and in Zhang et al., JMRI, vol. 14:pp. 56-62, 2001. These requirements reduce flexibility in choosing image parameters and increase scan time, which are both disadvantageous limitations.
Optical navigation is an alternative to the use of MR micro coils for the purpose of actively tracking and visualizing an interventional device. Active device tracking using optical systems is based on the principle of optical triangulation, such as discussed in Bernays et al., Neurosurgery, vol. 46(1): pp. 112-116, 2000. Unfortunately, these optical systems can only be used on open MR systems that provide a free optical access path to the camera of the optical system. Furthermore, these optical techniques cannot be used to track guidewires, or other optically transparent instruments, placed within the patient's body.
In the art of medicine, techniques have been investigated and utilized for the purpose of catheter localization combined with x-ray supervision that use spatially varying external magnetic fields as a magnetic source of instrument localization. These magnetic navigation systems are, or have been, provided by different manufacturers such as CARTO EP Navigation, NOGA Navigation System, TELESTAR, NAVION, or Flock of Birds (See for example, Ben-haim et al., No. Med., vol. 2(12):pp. 1393-1395, 1996; Kornowski et al., Circulation, vol. 98(11):pp. 1116-1124, 1998; Starkhammer et al., Biomedical Instrumentation and Technology, vol. 30:pp. 164-170, 1996; Shpun et al., Circulation, vol. 96(6):pp. 2016-2021, 1997). Another magnetic navigation system has been described for guiding endovascular interventions under x-ray monitoring (See Tanase et al., Sensors and Actuators, vols. 97-98:pp. 116-124, 2002; and U.S. Pat. No. 6,427,314 B1 to Acker). These research groups have demonstrated that Hall sensor-based navigation, within an external field of up to 0.4 mT, is feasible up to a positional accuracy of 2-3 mm. However, these prior art magnetic navigation systems cannot be used within a MR environment.
The present invention endeavors to provide a new navigation system based on the measurement of the local and spatially dependent magnetic field generated within the MR magnet bore, which takes advantage of measurements of the highly precise, spatially varying magnetic field. The proposed method relies on a direct measurement of the local magnetic field by means of magnetically sensitive devices, such as Hall devices and the like, thereby providing measurements of the local magnetic field for determining the position of the interventional device in a manner that is totally independent of the simultaneously applied MR imaging process.
Accordingly, a primary object of the present invention is to overcome the disadvantages of the prior art methods and apparatuses, both passive and active, for tracking and visualizing interventional devices within the MR environment.
Another object of the present invention is to provide a method and apparatus for tracking and visualizing an interventional device in an MR environment.
Another object of the present invention is to provide a method and apparatus for tracking and visualizing an interventional device in both open and closed MRI scanners.
Another object of the present invention is to provide a method and apparatus for tracking and visualizing an interventional device in an MR environment by directly measuring local linear magnetic field gradients, but not via measuring the magnetic resonance signal.
Another object of the present invention is to provide a method and apparatus for tracking and visualizing an interventional device in an MR environment, wherein the method and apparatus reliably track and provide visualization of large or small interventional devices without using MR coils and other magnetic sensing devices that operate using Faraday's Law.